Conventionally, as a technique of sharing frequencies among a plurality of radio communication systems including a priority system and a non-priority system, such a technique is known that the non-priority system transmits a signal using a shared frequency band in accordance with the presence or absence of a signal of the priority system in the shared frequency band (for example, Japanese Unexamined Patent Publication No. 2006-222665).
More specifically, the non-priority system determines whether there is a signal from the priority system in the shared frequency band before transmitting a signal from the non-priority system. To prevent an interference in the priority system from causing by a transmission signal from the non-priority system, the non-priority system transmits a signal using the shared frequency band only when a signal from the priority system is not present.
However, in the above-mentioned conventional technique, even when the load of the priority system is allowable, the non-priority system does not transmit a signal in the case where a signal from the priority system exits. Therefore, a problem arises that although the load of the priority system is allowable, a transmission opportunity of a signal from the non-priority system is restricted.
Further, to increase the transmission opportunity from the non-priority system in the above-mentioned conventional technique, it is considered that the non-priority system transmits a signal even when a signal from the priority system exists. However, in this case, when a receiver in the priority system is located near a transmitter in the non-priority system, an interference level in the receiver in the priority system rapidly increases by a transmission signal from the non-priority system. Therefore, there is a problem that signal quality deteriorates in the priority system.